1. Field of the Invention
The present invention relates to an exhaust apparatus in an ion implantation system for implanting impurities into a target substrate to be processed, and more particularly to an exhaust apparatus in an ion implantation system which can lengthen service life thereof and facilitate maintenance thereof.
2. Description of the Related Art
Conventionally, as an exhaust apparatus for an ion source chamber in an ion implantation system, a combination of an oil diffusion pump and an oil rotary pump has been used. However, because of contamination of the ion implantation system caused by back streaming of oil vapor, the necessity of exchange of degraded oil and safety concerns during the exchange process of the degraded oil as well as the problem of disposal of the degraded oil, such a wet exhaust apparatus which uses oil has been replaced, in recent years, with a dry exhaust apparatus which does not use oil and are made up of a combination of a turbomolecular pump and a dry-sealed vacuum pump (hereinafter referred to as dry pump). Conversion of the wet exhaust apparatus to the dry exhaust apparatus creates a new problem that exhausted substances, particularly solid substances which have been trapped by the oil in the pumps are discharged in large amounts because there is no medium which traps such substances. Typical examples of the substances which are implanted into a substrate to be processed by the ion implantation system are arsenic, boron and phosphorous derived from solid sources. Even if these substances are derived from gaseous sources such as arsine, phosphine and boron trifluoride, most of these substances take the form of simple substance in the ion source chamber, and thus a large amount of solid particles comprised of arsenic, boron or phosphorous are contained in the exhaust gas from the ion source chamber as in the case of solid sources.
There has been no report or information about the behavior of such substances, however, our own studies have revealed the following:
Since boron has a melting point of 2300.degree. C., boron takes the form of solid immediately in the ion source chamber, and although about 10% solid boron adheres to the inner wall of the ion source chamber, about 90% thereof is discharged from the system in the form of fine particles without adhering to the turbomolecular pump and the dry pump. As for arsenic, because the saturated vapor pressure of arsenic at a temperature of 160.degree. C. corresponds to an internal pressure of 10.sup.-5 Torr in the ion source chamber, about 50% of arsenic is sublimed and adheres to the inner wall of the ion source chamber, and the remaining 50% thereof is sublimed and deposited within the exhaust apparatus before reaching the atmospheric exhaust pipe of the dry pump. As for phosphorous, it is likely to be in the form of white phosphorous which has a fairly high vapor pressure even at room temperature, and white phosphorous is sublimated and adheres to both the inner wall of the ion source chamber and the pumps as well as the region in the downstream side of the atmospheric exhaust pipe.
In this way, by using the dry exhaust apparatus, arsenic, phosphorous and boron adhere to various regions in the ion implantation system, which causes the following problems:
First, the adhesion of the above substances to an insulating exhaust pipe causes discharge, frequently resulting in the malfunction of the ion implantation system in a short term.
Second, highly reactive substances such as white phosphorous adhere to the exhaust pipe in the downstream side of the vacuum pump and in the atmospheric condition, thus necessitating hazardous maintenance work.